The present invention relates to a charging and discharging control circuit for controlling the charging and discharging of a secondary battery by turning on/off a switch circuit and to a charging type power supply device of a secondary battery having the charging and discharging control circuit incorporated therein.
As a conventional charging type power supply device comprising a secondary battery, a power supply device shown in a circuit block diagram of FIG. 2 is known. Such a structure is disclosed in, for example, Japanese unexamined patent application publication No. H4-75430 (1992) entitled "CHARGING TYPE POWER SUPPLY DEVICE." More specifically, a secondary battery 101 is connected to an external terminal -V0 or +V0 through a switch circuit 103. Further, a charging and discharging control circuit 102 is connected in parallel to the secondary battery 101. The charging and discharging control circuit 102 has a function of detecting the voltage of the secondary battery 101. In case the secondary battery 101 is either in an over-charge state (a state where the battery voltage is above a predetermined value--hereinafter referred to as an over-charge protection state) or in an over-discharge state (a state where the battery voltage is below a predetermined value--hereinafter referred to as an over-discharge protection state), a signal is output by the charging and discharging control circuit 102 to turn off the switch circuit 103. Further, it is possible to restrict the current through the switch circuit 103 by controlling the switch circuit 103 so that, when the external terminal +V0 reaches a certain voltage, the switch circuit 103 turned off to stop the discharging. In other words, the discharging can be stopped when the current is excessive (over-current control). This state is hereinafter referred to as an over-current protection state. It is the role of the charging and discharging control circuit 102 to protect the battery against these states.
As another example of a conventional charging type power supply device comprising a secondary battery, a power supply device shown in a circuit block diagram of FIG. 3 is also known. The circuit shown in FIG. 3 differs from that shown in FIG. 2 in that the switch circuit 103 is connected in series with a negative electrode 111 of the secondary battery 101. In this way, the switch circuit 103 can function similarly in either case.